Waste-heat recovery system

ABSTRACT

A waste-heat recovery system for a waste-heat source includes an ORC (Organic-Rankine Cycle) postconnected thereto, the waste-heat source being in connection with the heating device of the ORC, as well as with an expansion machine, coupled to a generator, for steam expansion in the ORC, which has magnetic bearings with an associated control device and a power supply via a direct current intermediate circuit of a generator frequency converter. The unit made up of the expansion machine, the generator and the frequency converter is cooled by the coolant from the ORC circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a waste-heat recovery system.

2. Description of the Related Art

An ORC (Organic-Rankine Cycle) is a thermodynamic cyclic process according to Rankine. This means that a working medium runs through various thermodynamic states in order to be transferred back into the initial liquid state again at the end. In the process, the working medium is brought to a higher pressure level by a pump. Then, the working medium is preheated to evaporation temperature and subsequently evaporated.

Thus, it is an evaporation process, in which an organic medium rather than water is evaporated. The created steam drives an expansion machine, e.g., a turbine, a piston or propeller motor, which in turn is coupled to an electric generator in order to generate power. Downstream from the working machine, the process medium enters a condenser where it is cooled down again through heat dissipation. Since water evaporates at 100° C. under atmospheric conditions, it is frequently impossible to use heat at a low temperature level, e.g., industrial waste heat or earth heat, to generate power. However, if organic media with lower boiling point temperatures are used, then it is possible to generate low-temperature steam. ORC systems are also advantageous when used for exploiting biomass in connection with a combined generation of electricity and heat, especially at relatively low outputs, i.e., when the conventional biomass combustion technology seems relatively expensive. Biomass plants often have a fermenting device for the production of biogas, which normally has to be heated.

Generic waste-heat recovery systems are known from the field of combined power and heat generation and composed of a combined heat and power plant linked to a downstream ORC. The

German patent document DE 195 41 521 A1 describes a system for increasing the electrical efficiency in the generation of power from special gases by means of combustion engines, in which the waste heat of the engine is utilized for the further energy generation in a post-connected energy-conversion system. However, only the high-temperature heat from the cooling-water circuit as well as the exhaust-gas heat exchanger of the engine is provided for exploitation.

In addition, a diesel power unit integrated into a Rankine process is known from the U.S. Pat. No. 4,901,531, in which one cylinder is used for the expansion according to Rankine, and the other cylinders operate as diesel engine. U.S. Pat. No. 4,334,409 describes a system operating according to the Rankine process, in which the working fluid is preheated by a heat exchanger, through which the air from the outlet of a compressor of a machine having internal combustion is routed.

Block thermal power plants (BHKW) as plants for the cogeneration of electricity and heat are generally known. These are decentralized power generation plants, often driven by combustion engines, featuring simultaneous utilization of the waste heat. As far as possible, the heat withdrawn via the cooling media during combustion is used for heating suitable objects.

In particular as far as plants for the combined generation of power and heat having a post-connected ORC as waste heat power plant are concerned, machines based on engines having an exhaust-gas turbocharger for charging have come to dominate. That satisfies the demand for machines having very high electrical efficiencies, which are achievable only with turbocharging and recooling of the combustion-gas mixture heated by the condensation. Cooling of the combustion-gas mixture is generally required because the charge of the cylinder would otherwise be relatively poor. The cooling increases the density of the aspirated mixture, and the volumetric efficiency is improved. The output yield and the mechanical efficiency of the engine increase as a result.

Engine manufacturers stipulate a cooling-water intake temperature of only approximately 40 to 50° C. for the mixture cooling to allow sufficient cooling of the mixture. Since this temperature level is relatively low, the heat extracted from the combustion-gas mixture in the systems for the combined generation of power and heat known heretofore is dissipated to the environment, e.g., using a table-type cooler.

In addition, the preheating of the working medium in the ORC in two steps in a heating device is known from German patent document DE 10 2005 048 795 33, i.e., that the process medium in the ORC is heated by two heat exchangers connected in series downstream from the feeding pump, the first heat exchanger downstream from the feeding pump being provided as the first stage for the incoupling of low-temperature heat, and the following heat exchanger being provided as the second stage for the incoupling of high-temperature heat. Via a circulation system, the mixture cooling of the combustion engine is connected to the first heat exchanger downstream from the feeding pump, the heat from the cooling of the combustion-gas mixture aspirated by the combustion engine being used to preheat the process medium in the ORC and coupled into the first heat exchanger in the form of low-temperature heat. A second heating circuit obtains heat from the engine cooling water and exhaust gas of the internal combustion machine and is connected to the second heat exchanger downstream from the feeding pump, the heat from the cooling circuit and the exhaust gas being used to overheat and evaporate the process medium in the ORC and being input into the second heat exchanger downstream from the feeding pump in the form of high temperature heat.

BRIEF SUMMARY OF THE INVENTION

Therefore, the present invention is based on the objective of optimizing the design and safe operating behavior of a waste-heat recovery system made up of an ORC post-connected to a waste-heat source.

The waste heat recovery system is made up of, among other components, an expansion machine for vapor expansion in the

ORC, which has magnetic bearings with an associated control device and a power supply via a direct current intermediate circuit of a generator frequency converter. The waste heat recovery system is characterized by a unit which is cooled by the coolant from the ORC circuit and made up of expansion machine, generator and frequency converter. To achieve this, cool liquid coolant is extracted downstream from the feeding pump in the present invention and conveyed for cooling purposes to the unit made up of expansion machine, generator and frequency converter. In one especially advantageous specific development, the cool liquid coolant is removed downstream from the feeding pump and directly forwarded to the expansion machine to cool the bearing.

In addition, in the present invention heated coolant emerging from the unit made up of expansion machine, generator and frequency converter and/or from the bearing region of the expansion machine is supplied to the condenser on the intake side.

For example, temperature ranges of the coolant used for cooling of approximately 15° C. to 50° C. on the intake side, and approximately 30° C. to 80° C. on the discharge side are involved, the particular temperatures depending on the current operating state of the component parts and/or subassemblies to be cooled, as well as the overall waste heat recovery system.

In an advantageous manner, a temperature-monitoring device is provided which is linked to a superposed control device having temperature measuring points in the component parts and/or subassemblies to be cooled. This temperature-monitoring device compares current measured temperature values to predefinable setpoint values, analyzes them and/or optimally controls the coolant throughput accordingly. Preferably, separate control circuits having separate cooling channels or corresponding lines are provided for the component parts and/or subassemblies to be cooled. These individual control circuits assigned to the various component parts and/or subassemblies to be cooled have valves, preferably magnetic valves, to control the coolant throughput, so that the particular local temperature situation is able to be managed in optimal manner.

Using the present invention, the design and the operating behavior of a waste-heat recovery plant, which is composed of an ORC downstream from a waste-heat source, is optimized.

Waste-heat sources may be, for example, combined heat and power plants, industrial plants or boiler plants.

The waste heat recovery system, especially the unit made up of expansion machine, generator and frequency converter, is optimally cooled by the measures according to the present invention, in a manner that takes the current situation into account. This is a prerequisite for a safe, robust system operation, on the one hand, and also for an effective and careful operation of the individual components, on the other, each component having particular requirements with regard to cooling. This applies not only to the steady-state operation of the waste heat recovery system, but also to the modulation of the system in accordance with the waste heat volume as well as the start-up and shutdown. These states, in particular, constitute a challenge for the cooling system and are able to be managed in a safe manner according to the present invention.

In the start-up phase, for example, maximum operating safety and protection from coolant condensation are achieved if the run-up of the expansion machine, which is linked to the motor-operated generator, takes place without any coolant application in the ORC circuit. Since the partial coolant flow that is used for this purpose on the coolant side is routed via the generator unit, it absorbs the heat which is produced there by mechanical losses during motor-actuated operation. The cooling medium thereupon flows through the housing of the expansion machine, releases heat there and in this way initially provides preheating during the start-up phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows the schematic structure of a waste-heat recovery plant made up of an ORC post-connected thereto.

DETAILED DESCRIPTION OF THE INVENTION

The components that are important for operating the ORC are an ORC circulation circuit 1, a feeding pump 2, an evaporator 3, an expansion machine 4 for vapor expansion, which is coupled to a generator 5, a condenser 6 for recooling via a heat sink 7, and heat exchangers 8, 9 for preheating the working medium in ORC circulation system 1.

The two heat exchangers 8, 9 are connected in series downstream from feeding pump 2. First heat exchanger 8 downstream from feeding pump 2 is used as a first stage for the incoupling of low-temperature heat, and following heat exchanger 9 is used as a second stage for the incoupling of high-temperature heat from a waste-heat source 10.

A second heat circuit 11 is connected via its supply region to evaporator 3 of the ORC, because the temperature level initially is sufficiently high for its direct heating. Then, second heating circuit 11 discharges into second heat exchanger 9 on the return side, where is releases still existing residual heat to the ORC.

A liquid partial coolant flow 12 for cooling the expansion machine 4 is rerouted and first guided through generator 5.

Then, the cooling medium flows through the housing of expansion machine 4, where it initially releases heat for preheating in the start-up phase and ensures sufficient heat dissipation during normal operation. Only a simplified, schematic line design, without the required branching points to the individual component parts or subassemblies, sub-circuits, temperature-measuring points, valves and control devices are depicted in the drawing in this context.

Once a minimum starting speed has been reached, a steam valve 13 at the intake of expansion machine 4 is opened for steam expansion in the ORC, and during the further opening of steam valve 13, a further run-up of the engine speed takes place so that generator 5 transitions from motor-actuated operation to normal generator operation.

A controlled bypass 14 having at least one throttle valve is provided around expansion machine 4. This bypass 14 is initially open in the start-up phase, i.e., at a still relatively low temperature of the working medium. The working medium is routed around expansion machine 4 in this way. As soon as ORC circuit 1 has reached its setpoint operating state, throttle valve 15 in bypass 14 is closed, and steam valve 13 upstream from expansion machine 4 is opened. 

1-7. (canceled)
 8. A waste-heat recovery system implementing an ORC (Organic-Rankine Cycle) for a waste-heat source, comprising: a heating device in connection with the waste-heat source; a generator; an expansion machine coupled to the generator, wherein the expansion machine is in connection with the waste-heat source and configured for steam expansion in the ORC, and wherein the expansion machine has magnetic bearings with an associated control device and a power supply via a direct current intermediate circuit of a generator frequency converter; and an ORC circulation conduit channeling a coolant; wherein a unit which is cooled by the coolant from the ORC circulation conduit is formed by the expansion machine, the generator, and the frequency converter.
 9. The waste heat recovery system as recited in claim 8, further comprising: a feeding pump connected to the ORC circulation conduit channeling a coolant, wherein a cool, liquid coolant is withdrawn downstream from the feeding pump and supplied for cooling to the unit formed by the expansion machine, the generator, and the frequency converter.
 10. The waste heat recovery system as recited in claim 9, wherein the cool, liquid coolant is withdrawn downstream from the feeding pump and conveyed to the expansion machine for cooling the bearings.
 11. The waste heat recovery system as recited in claim 9, further comprising: a condenser on an intake side, wherein a heated coolant emerging from the unit formed by the expansion machine, the generator, and the frequency converter is conveyed to the condenser.
 12. The waste heat recovery system as recited in claim 9, further comprising: a temperature monitoring device linked to a superposed control device, wherein the temperature monitoring device (i) measures temperature values at points within the unit which is cooled, (ii) compares the measured temperature values to specified setpoint values, and (iii) controls a throughput of the coolant as a function of a result of the comparison.
 13. The waste heat recovery system as recited in claim 12, wherein separate control circuits are provided for component elements of the unit which is cooled, in order to control the throughput of the coolant.
 14. The waste heat recovery system as recited in claim 13, wherein valves for control of the throughput of the coolant are provided in the component elements of the unit which is cooled. 